1. Technical Field
This invention relates to transistor switches in general and more particularly to boot-strapped field effect transistor switches.
2. Background Information
Field effect transistors can be used as switches, for example when CMOS (Complementary Metal Oxide Semiconductor) technology is employed. The source and drain terminals of a field effect transistor then form the input and output terminals of a switch, while the gate terminal of the field effect transistor is a control terminal of the switch. However, field effect transistors have non-idealities which, for example, will cause the on-resistance of the switch to vary depending on the applied voltages. Furthermore transition effects may occur when the switch changes its state.
A problem with field effect transistors is the voltage dependent on-resistance. A field effect transistor used as a switch has a non-zero on-resistance Ron, which can roughly be approximated as:
                              R          on                =                  1                      KP            ·                          W              L                        ·                          (                                                V                  G                                -                                  V                  T                                -                                                                            V                      S                                        +                                          V                      D                                                        2                                            )                                                          (        1        )            
where KP is the product of the mobility μ of the charge carriers and the oxide capacitance Cox, W and L are the width and the length of the channel region, respectively, and VS, VD, VG and VT are the source voltage, the drain voltage, the gate voltage and the threshold voltage, respectively. According to equation (1) the on-resistance Ron is a function of the source voltage VS, the on-resistance Ron depends on the input voltage Vin.
Another problem with field effect transistors is the dependency of the threshold voltage VT on the bulk-source voltage VBS. This effect can be approximated as:VT=VT0±γ·(√{square root over (2·|φF|−VBS)}−√{square root over (2·|φF|)})  (2)
where γ is a technology constant which depends on the used process and φF is the Fermi level.
As equation (2) is a function of the source voltage VS, the threshold voltage VT depends on the input voltage Vin. According to equation (1), this also influences the on-resistance Ron of the switch.
Another non-ideality of field effect transistors is charge injection. Charge injection is a transition effect, which will distort the input and output voltages of the switch when the switch turns off. When a field effect transistor turns off, the charge that has been built up in the channel must disappear. This charge will divide between the source and drain side, depending on the total capacitance at these terminals. The effect ΔV on the source voltage VS of the switch is approximated by equation (3). Parameter A is dependent on the total capacitance of the source and drain terminals of the field effect transistor.
                              Δ          ⁢                                          ⁢          V                =                  A          ·                                                    C                ox                            ·              W              ·              L              ·                              (                                                      V                    G                                    -                                      V                    S                                    -                                      V                    T                                                  )                                                                    C                GS                            +                              C                BS                            +                              C                sample                                                                        (        3        )            
wherein Cox, CGS, CBS and Csample are the oxide capacity, the gate-source capacity, the bulk-source capacity and the loading capacitance of the switch when it is used in a sample-and-hold structure, respectively, and 0<A<1.
Another transition effect, which distorts the source and drain voltages of the switch when the switch turns off, is clock-feedthrough. The parasitic gate-source capacitance CGS of the transistor switch, together with the load capacitance at the source form a voltage divider between the clock signal and the output terminal. This results in feedthrough of the control signal driving the switch. This effect can be approximated as:
                              Δ          ⁢                                          ⁢          V                =                                            C              GS                                                      C                GS                            +                              C                BS                            +                              C                sample                                              ·                      (                                          V                                  G                  ,                  off                                            -                              V                                  G                  ,                  on                                                      )                                              (        4        )            
where VG,off and VG,on are the gate voltages when the switch is turned off and on, respectively.
A known solution for the non-linear on-resistance Ron (see equation (1)) of a transistor switch is bootstrapping. Bootstrapping makes the gate-source voltage VGS of the switch constant during the sampling phase, resulting in a signal independent on-resistance Ron. Bootstrapping is implemented, for example, by applying a constant voltage, for example a supply voltage Vdd, between the source and gate terminals when the switch is turned on.
A disadvantage of the bootstrapping technique is that the gate voltage VG is boosted to a certain value above the source voltage VS, which may result in reliability problems.